Post-transcriptional methylation of nucleic acids will be studied in hamster and rat cell lines transformed by adenoviruses 2, 7 and 12; in 3T6 cells infected with murine sarcoma virus; in 3T3 cells transformed by SV40 virus; and in KB cells treated with chemical carcinogens. Analysis of specific methylation parameters, achieved by application of our new methods, will be used to determine (a) the relationship between synthesis and methylation of selected RNA populations, (b) the extent of methylation per RNA molecule synthesized, (c) the composition of methylated constituents of purified RNA preparations and (d) in vitro tRNA methylase activities in the above systems.